An investigation of whether septal gamma-aminobutyrate-containing interneurons are involved in the reduction in the turnover rate of acetylcholine elicited by substance P and beta-endorphin in the hippocampus

Septo-hippocampal drug interactions in post-trial memory processing

To determine if serotonin and GABA regulate post-trial memory processing of the cholinergic projection from the septum to the hippocampus, mice were trained on footshock avoidance in a T-maze. Immediately after training, drugs were injected into the septum, hippocampus or both. Retention was tested 1 week after training and drug administration. Ketanserin, a serotonin type 2 receptor antagonist at a dose of 0.5 ng, had no measurable effect on retention, but it reduced the dose of bicuculline, in the septum, or arecoline in the hippocampus that was needed to improve retention. DOI, a serotonin type 2 receptor agonist at a dose of 2.5 ng, had the opposite effect of increasing the doses of bicuculline and arecoline needed to improve retention. Bicuculline, a GABA(A) receptor antagonist at a dose of 0.1 pg, did not affect retention when injected alone into the septum, but it reduced the dose of arecoline needed to improve retention in the hippocampus. Muscimol, a GABA(A) receptor agonist at a dose of 5 ng, injected into the septum, increased the dose of arecoline needed to improve retention. The results of this study are compatible with models that propose that serotonin innervation from the median raphe drives GABA interneurons in the medial septum that synapse on cholinergic neurons projecting to the hippocampus.

Reduced dorsal hippocampal glutamate release significantly correlates with the spatial memory deficits produced by benzodiazepines and ethanol

Memory deficits frequently occur after taking benzodiazepines and ethanol. We studied in vivo hippocampal presynaptic glutamate transmission in conjunction with memory deficits induced by benzodiazepines and ethanol in rats as an animal model of amnesia. These drugs potently impaired spatial memory formation as evaluated by the Morris water maze task, the rank order among tested treatments being the combination of triazolam (20 micrograms/kg) with ethanol (2 g/kg) > or = triazolam (100 micrograms/kg) > ethanol (2 g/kg) > or = triazolam (20 micrograms/kg) > rilmazafone (20 micrograms/kg). On the other hand, these drug treatments also reduced glutamate release in the dorsal hippocampus but not in the cerebellum measured by microdialysis: the combined administration of triazolam with ethanol potently inhibited glutamate release to 60% of basal output in the dorsal hippocampus. These decreases in hippocampal glutamate transmission closely correlated with the extent of impairment of spatial memory performance (r = 0.990). Thus, the present results strongly indicated that presynaptic dysfunction in dorsal hippocampal glutamatergic neurons would be critical for spatial memory deficits induced by benzodiazepines and ethanol.

Microinjection of GABA-A agonist muscimol into the dorsal but not the ventral hippocampus impairs non-mnemonic measures of delayed non-matching-to-position performance in rats

Anatomical studies of afferent and efferent connections suggest that the hippocampus may have more than one information processing role that varies along the septo-hippocampal axis. The present study was conducted to test whether a distinct functional specialization for the dorsal versus ventral extents of the hippocampus could be detected. The effects of a GABA-A receptor agonist, muscimol, microinjected into either the dorsal or ventral hippocampus on an operant, spatial delayed non-matching-to-position (DNMTP) task were measured. A decrease in the number of trials completed per session and disruption of several DNMTP discrimination parameters were produced by muscimol microinjection into the dorsal hippocampus but not into the ventral hippocampus. Muscimol injected into either site did not impair the measure of working memory, delayed choice accuracy. These results are consistent with the view that hippocampal function varies along the septo-temporal axis, and that the dorsal hippocampus is relatively more critical to visual discrimination performance than the ventral hippocampus.

Chronic morphine increases hippocampal acetylcholine release: possible relevance in drug dependence

Previous studies have shown that cocaine and amphetamine stimulate acetylcholine release in the hippocampus via an action of endogenously released dopamine on dopamine D1 and D2 receptors. The present study was aimed at clarifying if the property of stimulating hippocampal acetylcholine release was shared by morphine. The acute administration of morphine (10 mg/kg i.p.) failed to modify acetylcholine release in the hippocampus. However, after repeated administration (10 mg/kg i.p. twice daily) morphine acquired the ability to stimulate hippocampal acetylcholine release. Thus, at days 5 and 7 of chronic morphine treatment, a challenge dose of morphine (10 mg/kg i.p.) increased acetylcholine release by 50 and 100%, respectively. Concomitantly with the development of the stimulant property on acetylcholine release, morphine also acquired that of producing behavioural stimulation and lost that of producing sedation and catalepsy. The morphine-induced increase in acetylcholine output was suppressed by the blockade of dopamine D1 receptors with SCH 23390 (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4, 5-tetrahydro-1H-3-benzazepine) (0.1 mg/kg s.c.), which also suppressed the morphine-induced motor stimulation. Moreover, repeated morphine administration markedly potentiated the stimulant effect of the dopamine D1/D2 receptor agonist apomorphine (R(-)-10, 11-dihydroxyaporphine) (0.1 or 0.5 mg/kg s.c.) both on hippocampal acetylcholine release and on behaviour. These results may suggest that the enhancement of hippocampal acetylcholine release as well as the development of behavioural sensitisation after chronic morphine could be related to the development of dopamine receptor supersensitivity. Moreover, increased acetylcholine transmission in the hippocampus may play a role in the 'memory' of the rewarding effects of drugs of abuse.

Medial septal injection of naloxone elevates acetylcholine release in the hippocampus and induces behavioral seizures in rats

The effects of injections of naloxone, a universal opioid receptor antagonist, into the medial septal nucleus on hippocampal acetylcholine (ACh) release and behavior were investigated in freely moving rats by means of the microdialysis method. The injection of naloxone (2, 10 and 20 micrograms) produced a marked increase in hippocampal ACh release in a dose-dependent manner. These effects of naloxone were reversed by the post-injection of [D-Ala2, N-Me-Phe4, Gly-ol]-enkephalin (DAGO; 10 micrograms), an opioid mu receptor agonist. Furthermore, basal release of hippocampal ACh was significantly reduced by the injection of DAGO alone. It was also found that rats given an injection of naloxone showed an increase in motor activity and occasionally exhibited behavioral seizures. These effects of naloxone were also reversed by the post-injection of DAGO. The present results suggest that endogenous opioids ionically inhibit the activity of septo-hippocampal cholinergic neurons via mediation of mu opioid receptors in the medial septal nucleus. They also suggest that endogenous opioids modulate the incidence of seizures, at least in part, through opioid mu receptors in the medial septal nucleus.

Chronic postinjury administration of MDL 26,479 (Suritozole), a negative modulator at the GABAA receptor, and cognitive impairment in rats following traumatic brain injury

The present experiment examined the efficacy of postinjury administration of MDL 26,479 (Suritozole), a negative modulator at the gamma-aminobutyric acidA (GABAA) receptor that enhances cholinergic function, in attenuating spatial memory deficits after traumatic brain injury in the rat. Two experiments were performed. In the delayed-dosing experiment, rats received a moderate level (2.1 atm) of fluid-percussion brain injury and were tested in the Morris water maze 11 to 15 days following injury. These rats were injected with either 5 mg/kg (eight rats) or 10 mg/kg (eight rats) of MDL 26,479 60 minutes before each water maze test. Additional rats were injured and treated with saline (eight rats) or were surgically prepared but not injured (eight rats). In the second experiment, an early postinjury dosing procedure was followed. Rats were injured in the same manner but drug treatment began 24 hours after injury and continued daily through Day 15. Results indicated that the rats in the delayed chronic dosing regimen did not differ from the injured, saline-treated rats in their latency to reach the goal platform (p > 0.05). However, those treated chronically beginning 24 hours after injury had significantly shorter latencies than the injured, saline-treated rats (p < 0.05). These results suggest that administration of agents that enhance cholinergic function may be an appropriate strategy for promoting cognitive recovery when given after traumatic brain injury. Furthermore, prolonged treatment may be necessary to elicit beneficial effects.

Enhancement of acetylcholine release by flumazenil in the hippocampus of rats chronically treated with diazepam but not with imidazenil or abecarnil

The effects of long-term treatment (three times a day for 3 weeks) with pharmacologically active doses of the novel anxiolytics and anticovulsants abecarnil (0.5 mg/kg, IP) and imidazenil (0.5 mg/kg, IP) on basal hippocampal acetylcholine release in freely moving rats were compared with those of diazepam (3 mg/kg, IP). Challenge doses of diazepam, abecarnil, and imidazenil decreased the extracellular acetyl-choline concentration in the hippocampus by the same extent in animals chronically treated with the respective drug or vehicle. Moreover, the abrupt discontinuation of long-term treatment with diazepam, abecarnil, or imidazenil failed to affect hippocampal acetylcholine release during the first 5 days of withdrawal. In contrast, the acute administration of the benzodiazepine receptor antagonist flumazenil (1 mg/kg, IP) 2 days after diazepam withdrawal elicited a marked increase (65%) in acetylcholine release in the hippocampus. Flumazenil failed to induce the same effect 5 days after diazepam withdrawal or 2 or 5 days after discontinuation of long-term treatment with abecarnil or imidazenil. These results indicate that (i) the inhibitory effects of full (diazepam), partial (imidazenil), and selective (abecarnil) benzodiazepine receptor agonists on acetylcholine output in rat hippocampus are not affected by repeated drug administration; (ii) discontinuation of long-term treatment with each type of agonist does not affect hippocampal cholinergic mechanisms; and (iii) flumazenil increases acetylcholine release only in the hippocampus of rats chronically treated with diazepam. Together, these data further differentiate the pharmacology of benzodiazepine receptor full agonists from that of partial and selective agonists.

Relationship between morphine and etonitazene-induced working memory impairment and analgesia

An 8-arm radial maze task was used to assess the possible role of the opiate system in the spatial memory of the rat. Increasing doses of etonitazene (0.005-0.06 mg/kg i.p.) and morphine (2.5-100 mg/kg i.p.) significantly impaired performance in the working memory components of the task. For both drugs this impairment was linearly related to the log of the administered dose, and the log-dose relationships were parallel. The regression lines calculated for each parameter for both drugs were parallel thus allowing us to calculate the potency: etonitazene proved to about 1000 times more potent than morphine in terms of correct arm entries, the number of errors and the total time taken to complete the task. Moreover, the progressive cognitive impairment produced by both opiates was closely related to an increase in analgesic effect. Pretreatment with naloxone (5 mg/kg i.p.) completely antagonised the disruptive effect of the opiates on working memory. The importance of the mu subtype opiate receptor in cognitive processes is discussed.

The benzodiazepine receptor antagonist flumazenil increases acetylcholine release in rat hippocampus

The benzodiazepine receptor antagonist flumazenil (2.5-20 mg/kg i.p.) increased acetylcholine (ACh) release by up to 85% in the hippocampus of freely moving rats. In contrast, the benzodiazepine receptor full agonist diazepam (2.5-10 mg/kg i.p.) decreased ACh release up to a maximum of 45% in the same brain area. Injection of flumazenil (10 pmol) or diazepam (10 pmol) into the medial septum increased (95%) or reduced (50%), respectively, ACh release in the hippocampus. The maximum effect produced by those drugs was of the same magnitude as that observed after systemic injection. The changes in hippocampal cholinergic function elicited by activation and blockade of benzodiazepine receptors in the medial septum may thus play a crucial role in the alterations of the cognitive processes elicited by benzodiazepine receptor ligands.

Stimulation of both dopamine D1 and D2 receptors facilitates in vivo acetylcholine release in the hippocampus

The effect of selective D1 and D2 dopamine (DA) receptor agonists and of a mixed D1/D2 agonist on hippocampal acetylcholine (ACh) release was investigated. LY 171555 (0.5 and 1 mg/kg, i.p.), SKF 38393 (1 to 10 mg/kg, i.p.), CY 208-243 (0.2 mg/kg, i.p.) and apomorphine (0.5 to 2 mg/kg, i.p.), at doses stimulating rat behavior, were found to increase the output of ACh in the hippocampus. Maximal increase was observed after LY 171555 1 mg/kg, SKF 38393 10 mg/kg, CY 208-243 2 mg/kg and apomorphine 2 mg/kg (85, 90, 87 and 210%, respectively). The enhancement of ACh release induced by either SKF 38393 (10 mg/kg) or LY 171555 (1 mg/kg) was prevented by the blockade of D1 receptors with SCH 23390 (0.1 mg/kg, s.c.). Co-administration of maximally active doses of LY 171555 (1 mg/kg) and SKF 38393 (10 mg/kg) produced an additive effect (about 200%). In contrast to the findings with high doses, low, presynaptic doses of LY 171555 and apomorphine reduced ACh output. Maximal reduction was observed after 0.05 mg/kg for both drugs, (43 and 52%, respectively). These results show that activation of dopaminergic transmission either at D1 and/or at D2 receptors enhances ACh output in the hippocampus.

Inhibition of hippocampal acetylcholine release by benzodiazepines: antagonism by flumazenil

Diazepam (2.5-10 mg/kg i.p.) and midazolam (2.5-10 mg/kg i.p.) decreased acetylcholine release in the hippocampus of freely moving rats. This effect was antagonized by pretreatment with flumazenil (1 mg/kg i.p.). These results show that activation of benzodiazepine receptors reduces the in vivo release of acetylcholine in the hippocampus, suggesting that the septo-hippocampal cholinergic system, which has a major role in the regulation of cognitive functions, is under inhibitory control exerted by gamma-aminobutyrate (GABA) neurons.

Intraseptal injection of GABA and benzodiazepine receptor ligands alters high-affinity choline transport in the hippocampus

Injection of GABA and benzodiazepine (BDZ) agonists and antagonists into the medial septum produced bidirectional alterations in hippocampal high-affinity choline transport (HAChT). Male Sprague-Dawley rats were injected in the medial septum with either drug vehicle, a BDZ agonist, antagonist, or inverse agonist, or with a GABA-A or GABA-B agonist or antagonist and sacrificed 1 h later for assessment of HAChT in hippocampal synaptosomes. The GABA-A agonist muscimol, the GABA-B agonist baclofen, and the BDZ agonist chlordiazepoxide (CDP) produced dose-related decreases in HAChT 1 h following injection into the septum. The muscimol-induced decrease in HAChT was prevented by prior intraseptal injection of the GABA-A antagonist, bicuculline. Intraseptal injection of GABA-A (bicuculline) or GABA-B (2-hydroxysaclofen) antagonists did not alter HAChT, whereas the BDZ antagonist flumazenil (RO15,1788) and the BDZ inverse agonist methyl-beta-carboline-3-carboxylate (beta-CCM) increased this measure up to 30% in a dose-dependent manner. These results demonstrate that cholinergic neurons in the medial septum can be modulated in a bidirectional way through the pharmacological manipulation of GABA-A, GABA-B, and BDZ receptors. The potential functional and therapeutic consequences of these interactions are discussed.

Spontaneous alternation and inhibitory avoidance impairments with morphine injections into the medial septum. Attenuation by glucose administration

Peripheral glucose administration attenuates impairments produced by peripheral injections of the opioid agonist, morphine, on spontaneous alternation. Injections of opioid agonists directly into the medial septum also impair memory. The present experiments examined whether systemic and intraseptal glucose injections could attenuate deficits on spontaneous alternation and inhibitory avoidance in rats treated with intraseptal morphine. Morphine (3.95 nmol) injected into the medial septum significantly impaired performance on spontaneous alternation and inhibitory avoidance tasks. Both systemic (100 mg/kg, i.p.) and intraseptal (18.33 nmol) injections of glucose, administered concomitantly with intraseptal morphine, attenuated the morphine-induced impairments on these tasks in rats. These findings suggest that one brain region where glucose may act is the medial septum, possibly by releasing opioid inhibition of cholinergic activity.

GABAergic mediation of indirect transsynaptic control over basal and spatial memory testing-induced activation of septo-hippocampal cholinergic activity in mice

A neurochemical study of the transsynaptic interactions established between septal GABAergic interneurones and cholinergic septo-hippocampal neurones was conducted using mice. The effects of acute in vivo injections of either muscimol (20-500 ng/0.2 microliter), bicuculline (100 ng-1 micrograms/0.2 microliter) or saline vehicle (0.2 microliter) into the medial septum on septo-hippocampal cholinergic activity were evaluated using measures of hippocampal high affinity choline uptake at 30 min post-injection in two main groups of mice. The first (quiet control) remained in their home cages during the post-injection period whereas the second (active) were submitted, 10 min following injection to a 20-min period of spatial working memory testing in an 8-arm radial maze. Intraseptal injections of either muscimol or bicuculline produced significant (25-50%) inhibition of hippocampal cholinergic activity in quiet conditions (basal) as compared to intact or saline-injected mice. In the active groups, whereas memory testing induced significant cholinergic activation (+15-20%) in intact and saline injected mice at 30 s post-test no significant memory testing-induced activation was observed in either muscimol or bicuculline-injected mice at any dose. The role of septal GABAergic interneurones in the indirect transsynaptic control over the basal and activated states of septo-hippocampal cholinergic activity is discussed with respect to the concept that these complex neuronal interactions contribute to the physiological mechanisms involved in the modulation of working memory performance.

Muscimol infused into the medial septal area impairs long-term memory but not short-term memory in inhibitory avoidance, water maze place learning and rewarded alternation tasks

These experiments investigated the effects of injections of muscimol (1 or 5 nmol), administered into the medial septal area prior to training, on memory tested at different retention delays after training in 3 tasks: an inhibitory avoidance task, a one-trial place learning task, and a rewarded alternation task. In all 3 tasks, intraseptal injections of muscimol did not impair memory performance at short retention delays, but impaired memory at the longer retention delays. These findings are consistent with the view that GABAergic regulation of the septohippocampal cholinergic system plays a selective role in the establishment of long-term memory.

Increases in cerebral blood flow in rat hippocampus after medial septal injection of naloxone

BACKGROUND AND PURPOSE

In a previous study, we occasionally found that the rat given naloxone in the preoptic region develops behavioral seizures. In view of knowledge that the forebrain including the medial septal nucleus provides cholinergic projections to the hippocampal formation, the present study examined the effects of naloxone injected into the medial septal nucleus on the local blood flow in the hippocampus.

METHODS

A polyurethane-coated platinum electrode with a 1-mm bare tip for measurement of blood flow and a guide cannula made of stainless steel tube for naloxone injection were implanted chronically into the brain. The cerebral blood flow was measured by the hydrogen clearance method in freely moving rats.

RESULTS

The injection of 50 micrograms naloxone caused a significant increase in hippocampal blood flow, with its peak at 20 minutes. Twenty micrograms naloxone caused a similar increase, but 10 micrograms caused only a slight increase that peaked at 30 minutes, suggesting a dose-response of naloxone effect. Hippocampal blood flow was not changed after the injection of saline into the medial septal nucleus and after the injection of naloxone into the caudate nucleus.

CONCLUSIONS

Taken together with previous findings, the results suggest that endogenous opioids exert a decreasing effect on the local blood flow in the hippocampus, probably mediated by the magnocellular cholinergic neurons projecting to the hippocampus.

Naloxone modulates the behavioral effects of cholinergic agonists and antagonists

Peripheral glucose administration enhances memory in rodents and humans. Recent findings suggest that glucose may affect behavior, in part, by augmenting central cholinergic functions and by attenuating central opiate functions. The present experiments examined interactions between an opiate antagonist, naloxone, and cholinergic agents to determine whether the effects would parallel those found with glucose. Three behavioral measures were assessed: tremors, hyperactivity, and spontaneous alternation. Naloxone (1 mg/kg) significantly augmented tremors elicited by physostigmine (0.3 mg/kg). Naloxone (1 mg/kg) also attenuated increases in locomotor activity and impairments in spontaneous alternation performance elicited by scopolamine (1 and 3 mg/kg for activity and alternation measures, respectively). Thus, across three diverse measures, naloxone produced effects similar to those previously reported for glucose. These findings are consistent with the hypothesis that release of cholinergic activity from opiate inhibition may contribute to glucose effects on behavior.

The regulation of high-affinity choline uptake in vitro in rat cortical and hippocampal synaptosomes by beta-carbolines administered in vivo

The effects of several beta-carboline derivatives on sodium-dependent high-affinity choline uptake (HACU) were investigated in rat hippocampus and cerebral cortex. HACU was measured in synaptosomal preparations from these areas after in vivo administration of the drugs. The convulsant, picrotoxin (6 mg/kg), stimulated HACU in both hippocampal and cortical synaptosomes. The convulsant inverse agonist benzodiazepine, methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) (5 mg/kg) stimulated hippocampal but not cortical HACU. However, other inverse agonists, methyl-beta-carboline-3-carboxylate (beta-CCM) (20 mg/kg) and ethyl-beta-carboline-3-carboxylate (beta-CCE) (20 mg/kg), stimulated HACU in the cortex but not in the hippocampus. The partial inverse agonist, N-methyl-beta-carboline-3-carboxylate (FG-7142) (20 mg/kg), inhibited cortical HACU and had no effect on hippocampal HACU. The antagonist beta-carboline, 3-hydroxymethyl-beta-carboline (3-HMC) (20 mg/kg), had no effect on either cortical or hippocampal HACU. None of these drugs displayed any effect on HACU when they were incubated directly in vitro with synaptosomal preparations at concentrations up to 100 microM, suggesting their activity is not directly on the cholinergic nerve terminal. The results suggest that beta-carbolines regulate hippocampal and cortical cholinergic activity as do other GABAergic drugs. However, unlike diazepam, which depresses cholinergic activity in both the hippocampus and the cortex the beta-carbolines differentiate between the hippocampus and cortex in their action.

Muscimol injections in the medial septum impair spatial learning

These experiments examined the role of GABAergic systems in modulating septohippocampal cholinergic influences on learning. Microinjections of the GABA(A) agonist muscimol (0.5, 1.0 or 5.0 nmol) or physiological saline were administered (0.5 microliters) into the medial septum of rats via chronically implanted cannulae just prior to daily training in the Morris water maze spatial learning task. The animals received 3 training trials on each of 4 days. The escape latencies of rats trained with a submerged escape platform at a fixed location were significantly shorter than those trained with a randomly located platform. Rate of learning of the fixed location was significantly impaired in rats given pretraining muscimol injections in the medial septum at doses (1.0 and 5.0 nmol) that significantly reduced hippocampal high-affinity choline uptake (HACU). Analyses of responses on a probe trial with no pretraining injections and no platform revealed that, in comparison with controls, animals that had received muscimol prior to each training session were likely to swim in the region where the platform had been located. The finding that muscimol-injected rats were subsequently able to learn the task when trained without muscimol injections indicates that the acquisition impairment was not due to a lasting effect of the drug injections. Our results are consistent with the view that the septal GABAergic modulation of the septohippocampal cholinergic pathway is involved in regulating the acquisition of spatial information.

Intraseptal administration of muscimol produces dose-dependent memory impairments in the rat

The present study examined the effects of intraseptal administration of the GABAergic agonist muscimol on performance of a radial-arm maze (RAM) task. Male Long-Evans rats were trained to perform a RAM task in which a 1-h delay was imposed between the sample and the test session. In this task rats have access to four out of eight maze arms during a predelay session. Following a 1-h delay, rats are returned to the maze and allowed to freely choose among all eight arms. Arms not blocked during the predelay session are baited, and entry into an arm chosen during the predelay session or a repeated entry into a postdelay chosen arm constitutes an error. Following acquisition, animals were implanted with a single cannula aimed at the medial septum. A within-subjects design was utilized to examine the effects of intraseptal administration of muscimol (0.0, 0.75, 1.5 or 3.0 nmol) on performance in this task. All drugs or artificial cerebrospinal fluid were administered immediately following the predelay session. Muscimol, a GABA-A agonist, produced a dose-dependent impairment in maze performance as evidenced by fewer correct choices in the first four postdelay choices and an increase in the number of errors. Intraseptal administration of muscimol did not significantly alter latency per choice on the RAM task nor did it affect locomotor activity levels. Muscimol-induced impairments were also observed when a 4-h delay was imposed between the fourth and the fifth maze selection, suggesting that the behavioral deficit represents an inability to store or retain spatial working memories rather than a general performance deficit. These data indicated that pharmacological manipulation of GABA-A receptors within the medial septum modifies working memory processes. The potential interaction of GABAergic and cholinergic mechanisms in the modulation of working memory processes is discussed.

Neuropeptides and septo-hippocampal neurons: electrophysiological effects and distributions of immunoreactivity

The septo-hippocampal neurons (SHNs), located in the medial septum, project to the hippocampal formation. The population of SHNs, as shown by single unit recordings in urethane-anesthetized rats, is heterogeneous, both in terms of patterns of spontaneous activity (a significant proportion of the SHNs display a characteristic rhythmically bursting activity at about 4 Hz) and of conduction velocity. Their average rate of spontaneous discharge is quite high (20 impulses per second). They are excited by the iontophoretic application of acetylcholine and various cholinergic agonists. They are also excited by some peptides such as substance P and TRH. Parallel studies in aged animals show that the physiological properties of the SHNs are altered, while their pharmacological properties seem to be unchanged. Immunohistochemical investigations using antibodies against various peptides and a monoclonal antibody against choline acetyltransferase (ChAT) show that SHNs retrogradely-labeled from the hippocampus often contain ChAT, less frequently galanin-like immunoreactivity and in a few cases enkephalin, luteinizing hormone-releasing hormone, or calcitonin gene-related peptide. In contrast, cholecystokinin, vasoactive intestinal peptide, substance P, somatostatin, dynorphin-B and neurotensin, although present in some medial septal neurons, were never observed in neurons projecting to the hippocampus.

Effects of acute and subacute administration of bicuculline on dopamine and muscarinic receptors in rat brain

The changes of characteristics of dopamine (D1 and D2) and muscarinic receptors after acute and subacute administration of bicuculline were examined in rat brain. The results demonstrated that in striatal region, the Bmax of D2 receptor was significantly increased with no change of affinity after subacute administration of bicuculline. However, neither Bmax nor Kd or the D2 receptor was changed after single administration. The characteristics of D1 receptor were not changed by either acute or subacute administration. The Bmax of muscarinic receptor in striatal area was significantly increased without changes of affinity after both a single and repeated administration of bicuculline. However, in hippocampus, the Bmax of muscarinic receptor was significantly decreased after subacute administration of bicuculline. The results suggest that the interactions among GABAergic, dopaminergic and cholinergic neuron exist in central nerve system.

Lack of GABAergic modulation of acetylcholine turnover in the rat thalamus

In order to investigate the GABAergic modulation of the cholinergic midbrain-thalamus pathway, the effects of the GABAA agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) and of the benzodiazepine, diazepam, on acetylcholine (ACh) turnover in the thalamus have been studied. TRACh was also measured in the frontal and parietal cortices of the same animals. Our data confirm an inhibitory GABAergic modulation of the cholinergic nucleus basalis-cortex projection but suggest a lack of GABAergic modulation of the cholinergic midbrain-thalamus pathway.

Temperature as a dependent variable in the study of cholinergic mechanisms

1. Change in core temperature over time can be used as a dependent variable when studying the effects of manipulations on neurotransmitter systems. This article focuses on the measurement of core temperature as a strategy for detecting changes in the status of cholinergic systems. 2. Cholinergic neurons participate in the process of thermoregulation and interventions affecting them alter the thermal response to cholinomimetics. For example, chronic treatment with amitriptyline, chronic swim stress and inescapable footshock supersensitize rats to the hypothermic effects of oxotremorine. 3. This is consistent with the hypothesis that the pathophysiologies of tricyclic antidepressant withdrawal phenomena and stress involve supersensitivity of muscarinic mechanisms. 4. Uses of thermoregulation paradigms for investigating the actions of lithium ion, electroconvulsive shock and substances of abuse on muscarinic mechanisms are discussed. Applications to problems in the arena of clinical research are highlighted.

Effects of intraseptal drug administration on pentobarbital-induced narcosis and hippocampal choline uptake

Effects of injection of drugs into the septum on pentobarbital anesthesia were investigated in the rat. Intraseptal microinjection of bicuculline (5 micrograms), arecoline (2 micrograms), and phenylephrine (5 micrograms) shortened, MK-212 (5 micrograms) prolonged, and atropine (2 micrograms) had no significant effect on the duration of pentobarbital-induced loss of righting reflex. Bicuculline and arecoline increased and MK-212 reduced hippocampal cholinergic activity as measured by change in hippocampal sodium-dependent high-affinity choline uptake after intraseptal drug injection. It is concluded that activation of the septal-hippocampal cholinergic pathway might be an important neuromechanism for recovery from pentobarbital-narcosis.

Modulation of the excitability of septohippocampal terminals in the rat: relation to neuronal discharge rate

The excitability of the axonal terminals of medial septal neurons projecting to the dentate gyrus has been studied in the anesthetized rat under various experimental conditions: spontaneous or drug-induced variations in neuronal soma discharge rate, conditioning stimulation of afferent pathways (perforant path, commissural pathway, fimbria-fornix). It has been observed that terminals excitability is inversely correlated to the level of neuronal ongoing activity. These effects were observed on virtually all septal neurons projecting to the dentate gyrus. Since about one half of the septohippocampal neurons are likely to be cholinergic, it follows that such a phenomenon is not transmitter specific.

Effects of GABAergic drugs in vivo on high-affinity choline uptake in vitro in mouse hippocampal synaptosomes

The effects of several gamma-aminobutyric acid (GABA)-ergic drugs on sodium-dependent high-affinity choline uptake (HACU) were investigated in the hippocampus. HACU was measured in vitro after in vivo administration of the drug to mice. HACU was inhibited by those drugs that enhance GABA transmission. The convulsant 3-mercaptopropionic acid, which decreases GABA levels, stimulated HACU. From these results and previous findings, it appears that GABA mediates a tonic inhibitory effect on the septal-hippocampal cholinergic system.

The central pressor actions of substance P are inhibited by GABA

The influence of GABA-ergic stimulation on the centrally evoked pressor and tachycardic responses to substance P (SP) was investigated in conscious rats. Intracerebroventricular (i.c.v.) pretreatment with the potent GABA agonist muscimol attenuated the pressor responses to i.c.v. administered SP in a dose-dependent and reversible fashion. Inhibition was maximal 5-60 min after muscimol injection and lasted up to 3 h. In contrast, the increases in heart rate in response to i.c.v. administered SP were not consistently inhibited by muscimol. Central pretreatment with muscimol prevented the increase of plasma adrenaline but not of plasma noradrenaline in response to i.c.v. administered SP. Our results demonstrate that the GABA-ergic system can exert an inhibitory control on pathways mediating the central pressor actions of SP. The selective inhibition of adrenaline secretion by muscimol implies that suppression of sympathetic outflow to the adrenal gland may be involved as an important mechanism. In addition, our findings point to a dissociation between pathways mediating the pressor and tachycardic effects of SP.

Multiple mu opiate receptors

In addition to morphine-selective mu 2 and enkephalin-preferring delta sites, recent evidence supports the presence within the central nervous system of a common site with very high affinity for both enkephalins and opiates termed the mu 1 site. This concept of a common, very high affinity site for multiple neurotransmitters is a unique concept in neuropharmacology, differing from classical transmitter systems which possess multiple receptor classes for a single transmitter. This review will address both the biochemical and pharmacological evidence supporting the existence of this site.

Differential effects of ethyl (R,S)-nipecotate on the behaviors of highly and minimally aggressive female golden hamsters

The GABA uptake inhibitor ethyl (R,S)-nipecotate produces a dose-dependent suppression of aggression in highly aggressive hamsters but not in minimally aggressive ones. This suppression occurs at doses below those producing peripheral cholinergic effects; at the highest dose used it persists after these effects have dissipated. Doses sufficient to suppress aggression have no significant effect on grooming, locomotor activity and other behaviors but do affect sunflower seed acceptance. The differential effects of the drug on highly and minimally aggressive animals may indicate that their differences in aggression are due to differences in endogenous GABAergic activity. These results, together with previous evidence for parallel circadian variation in GABA uptake and aggressive behavior, suggest that GABA uptake may be an important endogenous regulator of aggression.

Intraseptal morphine potentiates pentobarbital narcosis and hypothermia in the rat

Morphine injected intraseptally in the amounts of 35 and 70 nmol prolonged pentobarbital-induced narcosis in the rat. Pentobarbital-induced hypothermia was also potentiated by intraseptal injection of 70 nmol of morphine. These effects were antagonized when morphine was injected together with naltrexone (29 nmol). Naltrexone injected by itself into the septum did not significantly affect pentobarbital-narcosis and hypothermia. It is concluded that activation of mu opioid receptors in the septal region could affect the actions of pentobarbital.

GABAergic control of the cholinergic projections to the frontal cortex is not tonic

The turnover rate of acetylcholine was measured in the frontal cortex of rats after either microinjection of bicuculline into the substantia inominata (the source of the cortical cholinergic innervation) or kainic acid lesioning of the nucleus accumbens (the source of GABAergic innervation of the substantia inominata). Neither treatment affected cortical acetylcholine metabolism, suggesting that the GABAergic inhibition of the substantia inominata-cortical cholinergic pathway is not tonic.

Behavioral and neurochemical differentiation of specific projections in the septal-hippocampal cholinergic pathway of the rat

In the rat, an intraseptal injection of the gamma-aminobutyric acid (GABA) agonist muscimol decreases the turnover rate of acetylcholine in the hippocampus and, during extinction of a food-reinforced lever-press response, increases extinction responding in a dose-dependent manner. Intraseptal beta-endorphin decreases the turnover rate of hippocampal acetylcholine through activation of septal GABAergic interneurons and increases extinction responding. On the other hand, intraseptal substance P, which decreases the turnover rate of hippocampal acetylcholine in a manner unrelated to septal GABAergic mechanisms, fails to increase extinction responding. The turnover rate of acetylcholine in various hippocampal regions after intraseptal injection of muscimol and substance P was also studied. Muscimol decreases the acetylcholine turnover rate only in the ventral hippocampus, whereas substance P decreases it only in the dorsal hippocampus. We hypothesize that a lowering in the cholinergic input to the ventral hippocampus is capable of increasing extinction responding, whereas a decrease in the input to the dorsal hippocampus is without such an effect. Hence, the cholinergic projections to the two hippocampal areas are modulated by different transmitter systems and have different physiological functions.

An immunohistochemical study on the location of GABAergic neurons in rat septum

Antisera against L-glutamate decarboxylase (GAD), the synthesizing enzyme of gamma-aminobutyric acid (GABA) were used to locate GABAergic neurons and nerve terminals in the septal complex of the rat by using the peroxidase-antiperoxidase method. Varying densities of immunoreactive terminals were observed in saline-treated rats but nerve cell bodies were only demonstrated after interventricular or intraseptal injections of colchicine. Small and medium-sized GAD-positive neurons were found in lateral septal nuclei, the largest number of these cells being in the pars dorsalis, and in the bed nucleus of the stria terminalis. Several GAD-immunoreactive neurons were located in the medial septal nucleus and the nucleus of the diagonal band of Broca (DB), where the cells were larger in the ventral than dorsal parts of the region. In the medial septal nucleus and in DB the GAD-positive cell bodies were distributed similarly to cholinergic neurons. Large GAD-positive neurons were also found in the septofimbrial nucleus. Intense immunoreactivity in nerve terminals was observed in the lateral septal nucleus, around the island of Calleja magna, between the DB and nucleus accumbens, and in the septofimbrial and triangular septal nuclei. In contrast, the medial septal nucleus, the DB, and the bed nucleus of the stria terminalis only showed weak to moderate immunoreactivity. These results provide direct morphological evidence for the presence of neurons capable of synthesizing GABA in septal nuclei. We suggest that there are two different GABAergic neuronal systems operating in the septum: a population of small cells in the lateral septal nucleus and a group of large cells in the medial septum and DB.

Gabaergic stimulation inhibits central actions of angiotensin II: pressor responses, drinking and release of vasopressin

In conscious rats, intracerebroventricular (i.c.v.) treatment with the GABA agonist muscimol (1-100 ng) suppressed the pressor responses to ANG II (100 ng i.c.v.) in a dose-dependent and reversible fashion. Treatment i.c.v. with GABA (1-500 micrograms) produced a similar but shorter inhibition. Inhibition of endogenous GABA degradation with amino-oxyacetic acid (AOAA, 30 mg/kg i.p.) markedly reduced the pressor responses to ANG II (10-1000 ng i.c.v.) with a recovery period of 24 h. This inhibition was reversed by the GABA antagonist bicuculline (1 microgram i.c.v.). Muscimol (100 ng i.c.v.) did not significantly attenuate the pressor responses to i.c.v. histamine (10 micrograms). Pretreatment with muscimol (100 ng i.c.v.) drastically reduced the drinking responses to i.c.v. ANG II (500 ng) and increased the latency to drink. Muscimol also suppressed drinking induced by carbachol (50 ng i.c.v.). Muscimol (10-1000 ng i.c.v.) inhibited the ANG II (100 ng i.c.v.)-induced release of AVP from the pituitary gland with complete suppression of the response at the highest dose. Our results demonstrate that the GABAergic system exerts an inhibitory control on pathways mediating the various central actions of ANG II, which appears to be most specific for the ANG II-induced pressor responses.

The transsynaptic regulation of the septal-hippocampal cholinergic neurons

There is not yet a complete understanding of the functional interactions among various septal nuclei which regulate hippocampal function. Nevertheless, much has been learned histologically and biochemically about the major connections of the distinct areas of the septal complex and the chemical character of some of these pathways. The cholinergic septal-hippocampal pathway serves as a well defined link between these two important structures of the limbic system. Acetylcholine turnover rates in the hippocampus have been shown to increase or decrease proportionally to the activity of the cholinergic neurons originating in the septum. Moreover, these turnover rates have been shown to be modulated by intraseptal injections of agonists or antagonists of various neurotransmitters or neuromodulators which are stored in various cell groups located in the septum. By coupling this biochemical approach with techniques to study the receptor organization, greater detail concerning the transmitter and cotransmitter interactions among the various neuromodulators can be obtained.

Mu opiate isoreceptors: differentiation with kappa agonists

In vivo pharmacological data support the concept of mu (mu) isoreceptors in the rat CNS. The mu 1 receptor mediates analgesia and regulation of cholinergic neurons. Naloxazone may be a specific antagonist of this receptor type. In contrast, the mu 2 receptor is responsible for respiratory depression and regulation of nigrostriatal dopaminergic neurons. In this case, the kappa agonists EKC and MR-2034 appear to be specific mu 2 antagonists.

GABAergic regulation of the substantia innominata-cortical cholinergic pathway

Neurochemical and immunohistocytochemical studies have demonstrated a nucleus accumbens-substantia innominata GABAergic pathway. The present authors' data with muscimol given parenterally, as well as local injections into the substantia innominata, further support an inhibitory function for this pathway in the regulation of the substantia innominata-cortical cholinergic projection.

The dimethylheptyl derivative of (-)-delta 8-tetrahydrocannabinol reduces the turnover rate of gamma-aminobutyric acid in the septum and nucleus accumbens

Accumulating evidence suggests that the cannabinoids exert their action to reduce the turnover rate of acetylcholine in the hippocampus by an action in the septum via inhibitory gamma-butyric acid (GABA) containing interneurons. In the studies presented here administration of the potent dimethylheptyl derivative of (-)-delta-tetrahydrocannabinol, which has previously been shown to reduce the turnover rate of acetylcholine in the hippocampus, reduces the turnover rate of GABA in the septum. A simple model in which cannabinoids transsynaptically activate inhibitory GABAergic septal neurons impinging on cholinergic septal neurons does not explain the data. A more complex model suggesting that inhibitory GABAergic septal interneurons innervate other inhibitory GABAergic septal interneurons has been hypothesized.

GABA antagonists enhance dopamine turnover in the rat retina in vivo

Previous results provided evidence for an inhibitory GABAergic influence on the dopamine neurons of the rat retina, without proving that endogenous GABA physiologically regulates the activity of these cells. We injected picrotoxinin intraocularly to dark-adapted rats and measured retinal dopamine turnover. Dopamine was analyzed radioenzymatically, and the decline in dopamine after alpha-methyl-p-tyrosine was used as an index of dopamine turnover. Picrotoxinin significantly stimulated dopamine turnover (P less than 0.05). In similar experiments with light-exposed rats picrotoxinin slightly enhanced dopamine turnover beyond that produced by light alone. Intraocular bicuculline methiodide produced similar results in dark-adapted and also in light-exposed rats. These data suggest that endogenous GABA tonically inhibits the activity of the retinal dopamine neurons in the dark and that there may be some remaining GABA tone on these cells in the light-exposed rat. Atropine pretreatment did not affect the picrotoxinin-induced activation of retinal dopamine turnover which negates the involvement of a cholinergic interneuron.